THE VASOMOTOR MECHANISM 341 



constrictor fibers at their terminations in the muscles of the 

 arterioles. The constant presence of this hormone in the blood is 

 doubtless an important factor in maintaining that degree of vaso- 

 motor tone upon which the well-being of the Body depends. The 

 substance adrenalin as used experimentally shows several striking 

 characteristics. In the first place a very small concentration of it 

 (one part in ten thousand), introduced into a capillary region, 

 brings about so strong a constriction in the immediate neighbor- 

 hood as to stop the flow of blood completely through that region. 

 It is possible thus to prevent troublesome bleeding in small 

 operations. The effect of adrenalin used in this way is, however, 

 very transient; repeated injections are necessary to maintain the 

 constricted state. Adrenalin injected into the general circulation, 

 even in small doses, causes a marked rise of arterial pressure due to 

 general vasoconstriction. This again is a very momentary effect. 



Vasodilator Nerves. We have already noticed, in the case of 

 the stomach, one method by which a locally increased blood- 

 supply may be brought about in an organ while it is at work, viz., 

 by inhibition of local vasoconstrictor fibers. Frequently, how- 

 ever, in the Body this is managed in another way; by efferent vaso- 

 dilator nerves which inhibit, not the vasoconstrictor center, but 

 the muscles of the blood-vessels directly. The nerves of the 

 skeletal muscles for example contain two sets of efferent fibers : one 

 motor proper and the other vasodilator. When the muscle con- 

 tracts in a reflex action or under the influence of the will both sets 

 of fibers are excited; so that when the organ is set at work its 

 arteries are simultaneously dilated and more blood flows through 

 it. But if the animal have previously administered to it such a 

 dose of curare as just to throw out of function the true motor- 

 fibers, stimulation of the nerve produces dilatation of the arteries 

 without a corresponding muscular contraction. Quite a similar 

 thing occurs in the salivary glands. Their cells, which form the 

 saliva, are aroused to activity by special nerve-fibers; but the 

 gland-nerve also contains a quite distinct set of vasodilator fibers 

 which normally cause a simultaneous dilatation of the gland- 

 artery, though either can be artificially stimulated by itself and 

 produce its effect alone. 



Since the effect of stimulating vasodilator nerves is the same as 

 inhibiting the constrictor mechanism we might ask why there 



